Global Navigation Satellite Systems (GNSS) have grown beyond GPS and GLONASS with the deployment of new designs including the Japanese QZSS, European Galileo, and Chinese COMPASS systems, all of which follow the same fundamental methods of operation and position determination based on RF radio wave ranging techniques. The use and availability of various corrections to aid in the accuracy obtained from a satellite in a GNSS have grown as well.
To improve the accuracy obtainable for positioning and navigation data using a GNSS receiver, various techniques are employed. Some of these involve algorithmic improvements internal to the devices, others involve aiding the device with externally provided information. At a broad level these latter items fall onto three areas: corrected or improved orbital information, clock drift information, and localized propagation effects from the ionosphere and troposphere.
To overcome the localized propagation effects specifically, many kinematic or stationary positioning applications make use of one or more additional GNSS receivers. In this method, a reference receiver placed at a well-known location, referred to as the reference station, receives the same satellite signals as a remote receiver referred to as the rover. The measurement information obtained by the reference station is then sent to the rover, and used, along with knowledge of the actual position of the reference station, to compute the measurement errors common to both the reference station and the rover. This process typically occurs in real time or near real time, but can also occur in post-processing, where the data may be stored and dealt with when convenient. And multiple rovers may connect to any given reference station via a variety of communications devices (both one way and two way).
By taking the difference between signals received both at the reference station and at the remote rover, common-mode errors are effectively eliminated. This facilitates an accurate determination of the rover's coordinates relative to the reference station's coordinates. When the reference station's coordinates are kinematic (such as the case of nearby operating rovers/vehicles exchanging data), this further facilitates an accurate determination of position of each vehicle relative to the other.
This general technique of differencing signals is known in the art as differential GPS (DGPS) or differential GNSS (DGNSS). When DGPS/DGNSS positioning utilizing one or more carrier phases is done in real time while the rover is semi-static or in motion, it is most often referred to as Real-Time Kinematic (RTK) positioning.
A common feature of all these techniques involves sending either correction or raw measurement data from the reference station to a GNSS rover receiver in the field. A constant stream of this information is then used by the rover to perform pseudo-range adjustments, double differencing, and/or various other techniques known to those skilled in the art to refine the positioning and navigation data calculated by the rover receiver. Loss of, or interruption of, this data stream can in turn be very disruptive to the rover unit. This commonly occurs when various wireless transmissions systems are used. Such systems suffer from the same RF propagation issues as the GNSS signals and may be blocked or suffer from corrupting effects of multipath on the received signal.
The rate of sending such corrections varies with the rover population and the capacity of the delivery media. A variety of delivery media have been successfully used over the years, including dedicated radio links, commercial broadcasting, government operated broadcasting, cellular technologies, paging, Wifi, DSRC, and geostationary and LEO satellite links. A common update rate is at 1 Hz, although faster rates are used when there is need. Lower update rates of 5 to 15 seconds are also found, typically due to limits of the transmission technology. In the USCG NDGPS system, data rates of 50, 100, or 200 bits per second limit both the signal content and the message repetition rates (5 to 10 seconds between updates). In many commercial systems a combination of compression and adaptive scheduling is used to conserve bandwidth. With the advance of widespread TCP/IP connections over wireless and the growth in NTRIP sites, access to many reference stations can be obtained over the Internet at locations well beyond their effective service areas.
There may be a dozen or more GNSS satellites simultaneously in view above the horizon at a given location and time. The general requirement is to provide differential corrections for all signals of each tracked satellite (both code and carrier for L1, L2, L5, and beyond in newer GNS systems). In some applications this may be relaxed if the body of rover devices to be supported does not, or will not, track a given signal of interest. Because the reference station cannot generally know which satellites are in view of each individual rover at any instant in time, it is customary to send information covering the entire sky down to a relatively low elevation mask. As the number of active GNSS satellites deployed continues to grow with the continued deployment of new systems like the European Galileo and the Chinese COMPASS, the bandwidth required to exchange all data necessary for differential corrections will continue to increase as well. Data rates of 15 kbs or more are required today to convey this information for GPS and GLONASS satellites, and even this can be prohibitive and problematic for various reasons.
The problem addressed herein is to generate and transmit the essential error correction information to the rover devices without using excessive bandwidth or being subject to immediate degrading when loss and corruption of one or more messages in a sequence occurs. There is also a general need for a method that can accommodate variations in update rates and which can allow the rover itself to select the exact point in time for which suitable data is needed.